1. Field of the Invention
The present invention pertains to photoimaging and, in particular, the use of photoresists (positive-working and/or negative-working) for imaging in the production of semiconductor devices. The present invention also pertains to novel fluorine-containing polymer compositions which are useful as base resins in resists and potentially in many other applications.
2. Description of Related Art
Polymer products are used as components of imaging and photosensitive systems and particularly in photoimaging systems such as those described in Introduction to Microlithography, Second Edition by L. F. Thompson, C. G. Willson, and M. J. Bowden, American Chemical Society, Washington, DC, 1994. In such systems, ultraviolet (UV) light or other electromagnetic radiation impinges on a material containing a photoactive component to induce a physical or chemical change in that material. A useful or latent image is thereby produced which can be processed into a useful image for semiconductor device fabrication.
Although the polymer product itself may be photoactive, generally a photosensitive composition contains one or more photoactive components in addition to the polymer product. Upon exposure to electromagnetic radiation (e.g. UV light), the photoactive component acts to change the rheological state, solubility, surface characteristics, refractive index, color, electromagnetic characteristics or other such physical or chemical characteristics of the photosensitive composition as described in the Thompson et al. publication supra.
For imaging very fine features at the submicron level is semiconductor devices, electromagnetic radiation in the far or extreme ultraviolet (UV) is needed. Positive working resists generally are utilized for semiconductor manufacture. Lithography in the UV at 365 nm (I-line) using novolak polymers and diazonaphthoquinones as dissolution inhibitors is a currently established chip technology having a resolution limit of about 0.35-0.30 micron. Lithography in the far UV at 248 nm using p-hydroxystyrene polymers is known and has a resolution limit of 0.35-0.18 nm. There is strong impetus for future photolithography at even shorter wavelengths, due to a decreasing lower resolution limit with decreasing wavelength (i.e., a resolution limit of 0.18-0.12 micron for 193 nm imaging). Photolithography using 193 nm exposure wavelength (obtained from an argon fluorine (ArF) excimer laser) is a leading candidate for future microelectronics fabrication using 0.18 and 0.13 xcexcm design rules. Photolithography using 157 nm exposure wavelength (obtained using an F2 laser source) may be used for future microelectronics fabrication using 0.100 xcexcm or less design rules. The opacity of traditional near UV and far UV organic photoresists at 193 nm and shorter wavelengths precludes their use in single-layer schemes at these wavelengths.
Some resist composition suitable for imaging at 193 nm are known. For example, photoresist compositions comprising cycloolefin-maleic anhydride alternating copolymers have been shown to be useful for imaging of semiconductors at 193 nm (see F. M. Houlihan et al, Macromolecules, 30, pages 6514-6534 (1997); T. Wallow et al, SPIE, Vol. 2724, pages 355-364; and F. M. Houlihan et al., Journal of Photopolymer Science and Technology, 10, No. 3, pages 511-520 (1997)). Several publications are focused on 193 nm resists (i.e., U. Okoroanyanwu et al, SPIE, Vol. 3049, pages 92-103; R. Allen et al., SPIE, Vol. 2724, pages 334-343; and Semiconductor International, September 1997, pages 74-80). Compositions comprising addition polymers and/or ROMP (ring-opening methathesis polymerization) of functionalized norbornenes have been disclosed (e.g., PCT WO 97/33198 (Sep. 12, 1997) to B. F. Goodrich). Homopolymers and maleic anhydride copolymers of norbornadiene and their use in 193 nm lithography have been disclosed (J. Niu and J. Frechet, Angew. Chem. Int. Ed., 37, No. 5, (1998), pages 667-670). No resist compositions for imaging at 157 nm have been disclosed.
Copolymers of fluoroolefin monomers and cyclic unsaturated monomers are known (U.S. Pat. Nos. 5,177,166 and 5,229,473 to Daikin Industries, Ltd.). These patents do not disclosure the use of these copolymers in any photosensitive compositions. Copolymers of certain fluorinated olefins with certain vinyl esters and vinyl ethers are known. For example, the copolymer of TFE with cyclohexanecarboxylate, vinyl ester (Japanese Patent Appl. JP 03281664 to Dainippon Ink and Chemicals) and with cyclohexyl vinyl ether (Japanese Patent Appl. JP 54046286 to Asahi Glass Co.) are known. Copolymers of TFE and vinyl esters, such as vinyl acetate, and use of these copolymers in photosensitive compositions for refractive index imaging (e.g. holography) are known (U.S. Pat. No. 4,963,471 to DuPont). All of the prior art on copolymers comprising fluorinated olefins and vinyl esters of formula CH2xe2x95x90CHO2CR and vinyl ethers of formulae CH2xe2x95x90CHOCH2R or CH2xe2x95x90CHOR, have R groups with a C:H ratio that is relatively low and which is less than 0.58.
Homopolymers and copolymers of certain monomers that possess properties of being both cyclic and fluorine-containing are known as well as their application as components in photodefinable fluorine-containing polymer films (Japanese Kokai No Hei 9(1997)-43856. There is no teaching in this reference of homopolymers or copolymers comprised of polycyclic comonomers as photoresist components. Furthermore, there is no teaching that the compositions disclosed in this Japanese Kokai could be used in photoresists with imaging effected at far UV wavelengths of less than 180 nm.
U.S. Pat. No. 5,665,527 discloses a process for generating a negative tone resist image by coating a silicon wafer with a copolymer resist solution of pentafluoropropyl methacrylate-t-butyl methacrylate in a solvent, and then exposing at 193 nm and developing with a carbon dioxide critical fluid.
xe2x80x94EP 0789 278 A2, published on Aug. 13, 1997, discloses a photoresist comprising a resin containing an alicyclic skeleton selected from general formulas 1 and 2 shown below, and a radiation-sensitive acid-generating agent: 
wherein n is 0 or 1, A and B represent independently a hydrogen atom, a halogen atom, a monovalent hydrocarbon group having 1 to 10 carbon atoms or a monovalent halogenated hydrocarbon group having 1 to 10 carbon atoms, and X and Y represent independently a hydrogen atom, a halogen atom, a monovalent hydrocarbon group having 1 to 10 carbon atoms, a monovalent halogenated hydrocarbon group having 1 to 10 carbon atoms or an acid-cleavable group, provided that at least one of X and Y is an acid-cleavable group. A photoresist pattern may be formed by exposing the photoresist to far UV radiation such as KrF excimer laser (wavelength of 248 nm) or ArF excimer laser (wavelength of 193 nm).xe2x80x94
There is a critical need though for other novel resist compositions for use at 193 nm or lower, and particularly at 157 nm, that have not only high transparency at these short wavelengths but also other suitable key properties, including good plasma etch resistance and adhesive properties.
The present invention in some embodiments comprises
(a) a fluorine-containing copolymer comprising a repeat unit derived from at least one ethylenically unsaturated compound characterized in that at least one ethylenically unsaturated compound is polycyclic and at least one ethylenically unsaturated compound contains at least one fluorine atom covalently attached to an ethylenically unsaturated carbon atoms; and
(b) at least one photoactive component;
wherein the fluorine-containing copolymer does not contain aromatic functionality but contains sufficient functionality to render the photoresist developable so as to produce a relief image, upon imagewise exposure to ultraviolet radiation having wavelength of xe2x89xa6365 nm.
In one preferred embodiment, at least one ethylenically unsaturated compound is selected from the group consisting of: 
wherein:
each of m and n is 0, 1 or 2, p is an integer of at least 3;
a and b are independently 1 to 3 except that a is not=1 when b=2 or vice versa;
R1 to R14 are the same or different and each represents a hydrogen atom, a halogen atom, a carboxyl group, a C1 to C14 secondary or tertiary alkyl carboxylate, a hydrocarbon group or a substituted hydrocarbon group;
R15 is a saturated alkyl group of about 4 to 20 carbon atoms, optionally containing one or more ether oxygens with the proviso that the ratio of carbon atoms to hydrogen atom is greater than or equal to 0.58;
R16 to R21 are each independently hydrogen atoms, C1 to C12 alkyls, (CH2)qCO2A, CO2(CH2)qCO2A or CO2A wherein q is 1 to 12 and A is hydrogen or an acid protecting group with the proviso that at least one of R18 to R21 is CO2A.
The present photoresist in other embodiments comprises
(a) a fluorine-containing copolymer comprising a repeat unit derived from at least one polycyclic ethylenically unsaturated compound having at least one atom or group selected from the group consisting of fluorine atom, perfluoroalkyl group, and perfluoroalkoxy group, characterized in that the at least one atom or group is covalently attached to a carbon atom which is contained within a ring structure and separated from each ethylenically unsaturated carbon atom of the ethylenically unsaturated compound by at least one covalently attached carbon atom; and
(b) at least one photoactive component;
wherein the fluorine-containing copolymer does not contain aromatic functionality but contains sufficient functionality to render the photoresist developable so as to produce a relief image, upon imagewise exposure to ultraviolet radiation having wavelength of xe2x89xa6365 nm.
In another embodiment, the invention is a process for preparing a photoresist image on a substrate comprising, in order:
(W) applying a photoresist composition on a substrate, wherein the photoresist composition comprises:
(a) a fluorine-containing copolymer comprising a repeat unit derived from at least one ethylenically unsaturated compound characterized in that at least one ethylenically unsaturated compound is polycyclic and at least one ethylenically unsaturated compound contains at least one fluorine atom covalently attached to an ethylenically unsaturated carbon atom, wherein the fluorine-containing copolymer contains sufficient functionality to render the photoresist developable so as to produce a relief image, upon imagewise exposure to ultraviolet radiation having wavelength of xe2x89xa6365 nm;
(b) at least one photoactive component; and
(c) a solvent;
(X) drying the the coated photoresist composition to substantially remove the solvent and thereby to form a photoresist layer on the substrate;
(Y) imagewise exposing the photoresist layer to form imaged and non-imaged areas; and
(Z) developing the exposed photoresist layer having imaged and non-imaged areas to form the relief image on the substrate.
In another embodiment, the invention is a process for preparing a photoresist image on a substrate comprising, in order:
(W) applying a photoresist composition on a substrate, wherein the photoresist composition comprises:
(a) a fluorine-containing copolymer comprising a repeat unit derived from at least one polycyclic ethylenically unsaturated compound having at least one atom or group selected from the group consisting of fluorine atom, perfluoroalkyl group, and perfluoroalkoxy group, characterized in that the at least one atom or group is covalently attached to a carbon atom which is contained within a ring structure and separated from each ethylenically unsaturated carbon atom of the ethylenically unsaturated compound by at least one covalently attached carbon atom, wherein the fluorine-containing copolymer does not contain aromatic functionality but contains sufficient functionality to render the photoresist developable so as to produce a relief image, upon imagewise exposure to ultraviolet radiation having wavelength of  less than 365 nm;
(b) at least one photoactive component; and
(c) a solvent;
(X) drying the coated photoresist composition to substantially remove the solvent and thereby to form a photoresist layer on the substrate;
(Y) imagewise exposing the photoresist layer to form imaged and non-imaged areas; and
(Z) developing the exposed photoresist layer having imaged and non-imaged areas to form the relief image on the substrate.
The developing step (Z) in the above processes can either be negative-working or positive working.
In another embodiment, the invention is a fluorine-containing copolymer comprising:
(i) a repeat unit derived from at least one ethylenically unsaturated compound containing at least one fluorine atom covalently attached to an ethylenically unsaturated carbon atom; and
(ii) a repeat unit derived from the group consisting of CH2xe2x95x90CHO2CR15, CH2xe2x95x90CHOCH2R15 and CH2xe2x95x90CHOR15, wherein R15 is a saturated alkyl group of about 4 to 20 carbon atoms, optionally containing one or more ether oxygens with the proviso that the ratio of carbon atoms to hydrogen atoms is greater than or equal to 0.58.
In still another embodiment, the invention is a fluorine-containing copolymer comprising:
(i) a repeat unit derived from at least one ethylenically unsaturated compound containing at least one fluorine atom covalently attached to an ethylenically unsaturated carbon atom; and
(ii) a repeat unit derived from a comonomer having the structure: 
xe2x80x83wherein R16 to R20 are each independently hydrogen atoms, C1 to C12 alkyls, (CH2)qCO2A, CO2(CH2)qCO2A or CO2A wherein q is 1 to 12 and A is hydrogen or an acid protecting group with the proviso that at least one of R18 to R20 is CO2A.
A key characteristic of the copolymers (and photoresists comprised of the copolymers) of this invention is the cooperative combination of polycyclic repeat unit(s) with the same or different repeat units that are fluorine containing and, furthermore, with all repeat units in the copolymers not containing aromatic functionality. The presence of polycyclic repeat units in the copolymers is critical in order for the copolymers to possess high resistance to plasma etching (e.g., reactive ion etching). Polycyclic repeat units also tend to provide a high glass transition temperature which is critical for maintaining dimensional stability in the resist films. The presence of repeat units that are fluorine-containing is critical in order for the copolymers to possess high optical transparency, i.e., to have low optical absorptions in the extreme and far UV. The absence of aromatic functionality in the repeat units of the copolymers is also required in order for the polymers to possess high optical transparency.
In certain embodiments of this invention, the fluorine-containing copolymer may be comprised of a repeat unit derived from at least one polycyclic ethylenically unsaturated compound having at least one atom or group selected from the group containing of fluorine atom, perfluoroalkyl group, and perfluoroalkoxy group, covalently attached to a carbon atom which is contained within a ring structure. Fluorine atoms, perfluoroalkyl groups and perfluoroalkoxy groups tend to inhibit polymerization of cyclic ethylenically unsaturated compounds by metal-catalyzed addition polymerization or metathesis polymerization when such groups are attached directly to an ethylenically unsaturated carbon atoms. Thus, it is critical in such cases that the at least one fluorine atom, perfluoroalkyl group and perfluoroalkoxy group by separated from each ethylenically unsaturated carbon atom of the ethylenically unsaturated compound by at least one covalently attached carbon atom. Furthermore, attaching the atom and/or group directly to a ring minimizes the presence of undesirable non-fluorinated aliphatic carbon atoms.
The copolymers of this invention surprisingly have balanced properties that are critical for imparting necessary properties to photoresist compositions for semiconductor applications. First, these copolymers have unexpectedly low optical absorptions in the extreme and far UV, including 193 nm and 157 nm wavelengths. Having copolymers with low optical absorptions is critical for formulating high photospeed resists wherein the major amount of UV light is absorbed by the photoactive component(s) and not lost due to absorption by the copolymer (matrix of the resist). Second, resists comprising the fluorine-containing polymers of this invention desirably exhibit very low plasma etch rates. This latter property is critical in affording high resolution precision resists that are required in semiconductor fabrication. Achieving simultaneously suitable values of these properties is particularly critical for imaging at 157 nm. In this case, ultra thin resists are needed for high resolution, but these thin resists must nevertheless be highly etch resistant such that resist remains on imaged substrates and protects areas of underlying substrate during etching.
The photoresist compositions of this invention have a particularly good balance of desirable properties, including high transparency to extreme, far, and near ultraviolet light, high plasma etch resistance, and projected high resolution characteristics suitable for microelectronics device fabrication using 0.18 and 0.13 xcexcm design rules. The photoresist compositions of this invention in particular have good optical transparency at 157 nm. The novel copolymers of this invention also have good properties including anticipated high transparency at 157 nm and other wavelengths in the UV.